The invention relates to a hydraulic control device for an automatic dual-clutch transmission which has a first clutch and a second clutch.
Dual-clutch transmissions have been known for a relatively long time and are preferably used in motor vehicles. A dual-clutch transmission generally has a first partial transmission and a second partial transmission which are each assigned a group of gears. Here, the first clutch separates the first partial transmission from an engine of the motor vehicle, while the second clutch serves to separate the second partial transmission from, or connect the latter to, the engine.
As a result of the considerable mechanical expenditure in actuating two clutches and shifting the gears of both partial transmissions, dual-clutch transmissions are usually automatic, that is to say both the actuation of the clutches and the shifting of the gears is performed by means of associated auxiliary drives which are actuated by means of preferably one hydraulic control device.
There is often no provision in automatic dual-clutch transmissions for the driver of the motor vehicle to directly mechanically influence the transmission of torque of the dual-clutch transmission. In some dual-clutch transmissions, only a parking lock can be mechanically operated by the driver of the motor vehicle.
For safety reasons, the hydraulic control device should preferably be designed such that the torque flow through the dual-clutch transmission can always be reliably interrupted in the event of a fault. This can be provided by opening the clutches. For this purpose, the clutches are placed in an unpressurized state, as long as said clutches are hydraulically operated clutches which are open in the unpressurized state (normally open clutch).
A result of the above requirement is that a torque flow should also be interrupted when two individual faults which can have a direct effect on one another occur in succession.
In addition to the above described requirement regarding the safety of the dual-clutch transmission, it is also necessary to meet the requirement for the best possible availability of the motor vehicle in which the dual-clutch transmission is installed. The requirement of good availability is generally met when, in the event of an individual fault, at least one forward gear and one reverse gear are available in order to enable the vehicle to be moved in two directions.
The requirements regarding safety and availability are often not met by known control devices for dual-clutch transmissions. It is known for example from DE 101 34 115 A1 to actuate the first clutch (this correspondingly also applies to the second clutch) by means of a hydraulic path which consists of a first switching valve and a first control valve which is connected in series with the switching valve. The first switching valve, which can be operated by means of an electrical control signal, places the first clutch in an unpressurized state in a rest position, and when said first switching valve is in a position which it assumes when it has a current applied to it, it connects the main pressure line to the first control valve which is controlled by means of an electrical control signal. If, for example, the first switching valve can no longer be switched out of its rest position, the first clutch can no longer be operated. If reverse driving is effected via said clutch, the motor vehicle is no longer available, according to the above requirement, in the event of one single fault (failure of the first switching valve).
On the other hand, the first clutch, as long as it requires an actuating pressure to be closed, can no longer be opened if both the switching valve can no longer be placed in the rest position and also the pressure or the volume can no longer be reduced sufficiently by the control valve. In this case, the first clutch remains closed and the torque flow in the dual-clutch transmission can no longer be separated.
The same also applies to the second clutch. The dual-clutch transmission has, for the second clutch, a second switching valve and a second control valve, the same relationships arising in a similar way to the first clutch.